Recently bought a flat in a 1910 tenement block, and as part of its
renovation I plan to move the central heating boiler some 50cm higher from
its current position so that I can integrate it into one of the new
kitchen's wall cabinets.

My question is:

How does one go about blocking the 4" hole that will be left from the
current flue? I am not particularly worried about the appearance from the
outside as it is not visible at all (no buildings across the road, etc) -
just want to make sure that the remains weather-proof..

PS: the wall is a cavity wall.